The invention relates to systems and methods for aligning a transmit antenna with a remote receiver, for example for aligning a ground terminal antenna with a satellite.
In any satellite communication system, aligning the antenna of a ground terminal to the satellite is a prerequisite to allow the system to operate normally. In C-band and Ku band satellite communications systems in which only downlink transmissions to user ground terminals occur, the dish antenna at the ground terminal can be aligned with the satellite by monitoring received signal strengths and frequency spectra to find the boresight direction of the dish antenna. The sensitivity of the received signal strength to the pointing accuracy is quite low such that a substantial pointing error will not substantially effect the strength of the signal captured. Because of this, accurate systems and methods for aligning the antenna in such satellite communications systems have not been required.
In bi-directional satellite communications systems in which transmissions occur on both the downlink and the uplink, the problem of aligning the antenna of a ground station to optimize signal capture at both the ground terminal and the satellite is a special issue compared with the above-discussed systems employing only downlink communications. This is because in bi-directional systems, typically transmissions are made on different frequencies and/or different bands for the uplink vs. the downlink. For example, uplink transmissions may be done on the Ka band while downlink transmissions may be done on the Ku band.
Generally, a satellite terminal has some sort of antenna tracking system which utilizes an alignment approach. The resolution of conventional alignment approaches will limit the pointing accuracy around 0.5 dB for the receiving signal. However, due to the difference of the antenna beamwidth between the Ku band and Ka band, a 0.5 dB pointing loss existing in the Ku band (downlink) will result in a loss of up to 3.4 dB in the Ka band signals transmitted on the uplink. Therefore, this traditional approach to antenna alignment (also called open loop alignment) cannot provide enough pointing accuracy for the uplink in such a system.
More specifically, this almost 3 dB of additional loss means the ground terminal must double its power to reach the satellite. The additional transmitted power not only wastes the ground terminal""s power, increases the system cost, reduces the system margin and degrades the equipment reliability, but also increases the interference to other satellite systems, which results in a difficulty in cooperating with them.
It would be advantageous to have an alignment system and method which facilitates more accurate alignment of the ground terminal antenna such that the loss due to pointing accuracy could be substantially reduced compared to that of traditional methods.
Embodiments of the invention propose a novel antenna alignment mechanism for the Ka/Ku band satellite ground terminal, and provide a preferred design of a peak search tool for the realization of this method. A key advantage of this invention is the reduction of the antenna uplink pointing loss, even up to 3 dB. Based on this potential 3 dB savings in ground terminal transmitting power provided by the invention, the cost of equipment can be substantially reduced. In systems with many thousands of ground terminals, the total savings may become very significant.
In the preferred embodiment, either a 3 dB margin can be obtained, or the system availability can be improved from 99.5% to 99.7%. This invention can be used in any frequency band satellite systems, e.g. c, Ku, Ka bands, etc. and more generally in satellite systems having different up/down frequency combination links.
In one broad aspect, the invention provides a method of aligning an antenna with a satellite for an alignment parameter. From the antenna, a test signal is transmitted over an uplink channel to the satellite. Over a downlink channel from the satellite is received a sequence of alignment accuracy indications for the test signal. On the basis of the sequence of alignment accuracy indications, adjustments to the antenna""s orientation for the alignment parameter are made.
Preferably, the alignment accuracy indications are processed to give an indication of whether a previous adjustment resulted in an improvement or a degradation in pointing accuracy. This processing might for example consist of determining if the received alignment accuracy indications have become stable, detecting if a non-trivial change in the quality since the adjustment has occurred, detecting if there is an improvement in the received alignment accuracy indications, and determining if a maximum alignment accuracy indication has been received. If the received alignment accuracy indications have become stable, a non-trivial change has been detected, an improvement in the received alignment accuracy indications has been detected, an indication is output indicating that the previous adjustment was a good one. Alternatively, if the received alignment accuracy indications have become stable, a non-trivial change has been detected, an improvement in the received alignment accuracy indications has not been detected, an indication is output indicating that the previous adjustment was a bad one. Finally, if the received alignment accuracy indications have become stable, and the maximum alignment accuracy indication has been received, an indication is output indicating that the method for the parameter is complete.
Typically, the uplink signals are sent from the antenna to a satellite which retransmits them to a hub ground station which generates the alignment accuracy indications and transmits these to the satellite which retransmits them to the antenna.
While the alignment accuracy indication is preferably some measure of signal strength such as signal-to-noise ratio or absolute signal strength, any suitable alignment accuracy indication may be used.
To determine when the method is complete for a given parameter, a maximum in the alignment accuracy indications is preferably detected by determining when a sequence of the adjustments have been made which resulted in an oscillation in the antenna""s orientation about a common position.
Preferably, a xe2x80x9cRIGHTxe2x80x9d or xe2x80x9cWRONGxe2x80x9d indication is generated after each adjustment to the antenna orientation, wherein xe2x80x9cRIGHTxe2x80x9d indicates that the previous adjustment resulted in an improvement in pointing accuracy and that another adjustment in the same direction should be made, and xe2x80x9cwrongxe2x80x9d indicates that the previous adjustment resulted in a degradation in pointing accuracy and that an adjustment in the opposite direction should be made.
In another embodiment, a peak searcher apparatus is provided which is adapted to facilitate aligning an antenna with a satellite for an alignment parameter by coordinating antenna adjustment steps made by an installer. The apparatus has an input adapted to receive a sequence of alignment accuracy indications each somehow indicative of alignment accuracy, a maximum alignment accuracy indication finder functional entity adapted to identify when a maximum alignment accuracy indication has been received in the sequence of alignment accuracy indications, a direction finder functional entity adapted to make a determination from the sequence of alignment accuracy indications whether a most recent adjustment step made by the installer has resulted in an improvement or degradation, and an indicator adapted to indicate to the installer the determination made by the direction finder functional entity.
The peak searcher is preferably adapted for use with or in combination with a ground terminal which generates a test signal, transmits the test signal through the satellite to a remote receiver, receives alignment accuracy indications made by the remote receiver on the basis of the test signal, and passes these alignment accuracy indications to the peak searcher apparatus.
The direction finder functional entity is preferably further operable to identify predetermined patterns in a most recent sequence of the determinations which are defined to indicate no further adjustments are required. For example, it might have a memory for storing the most recent sequence of the determinations, and logic for identifying the predetermined patterns.
More generally, an embodiment of the invention provides an alignment procedure for aligning any antenna with any remote receiver for an alignment parameter. The method involves transmitting from the antenna a test signal over a transmit channel to the remote receiver, receiving a sequence of alignment accuracy indications for the test signal, and making a plurality of adjustments to the antenna""s orientation for the alignment parameter on the basis of the sequence of alignment accuracy indications.